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Old 10-24-2012, 05:51 PM   #1 (permalink)
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4.30 gears and 35inch tires

I can't seem to find the perfect rpm range towing with 3.73 gears and 35inch tires. So I want to change gears. I have talked to several rear end shops and all of them recommended 4.30 gears. Just wanted to get some feed back from anyone running 35inch tires with 4.30 gears.
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Old 10-25-2012, 09:23 PM   #2 (permalink)
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Nobody has gears other than 3.73? I am trying to get real world data. I made my appt for Monday.
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Old 10-26-2012, 03:37 AM   #3 (permalink)
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I have 4.30 gears. Based on the fact that my truck came with 31.7 tires and 3.73 gears I installed 4.30 gears and slightly taller 32.7 inch tires, this now gives me roughly a 4.17 final drive.
Your 35s with 3.73 gears are giving you a current final ratio of 3.38. If you install 4.3 gears and keep the 35s you'll be running 3.90s. That would be a decent ratio. my next tire will be 33.4 which should alter my final drive to apx 4.10. IMHO anything around 4 to 1 is gonna have decent get up and go.
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Old 10-26-2012, 09:49 AM   #4 (permalink)
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The math is not hard. You can find tire size calculators on the internet, but I don't trust them to use the right math.

The change in percentage of the tire revolutions per mile is equal to the percentage of change in rear axle ratio.

Stock tires on your Y2K 4x4 PSD were LT265/75R16 with 655 revs/mile. 35s have about 600 revs/mile. Revs/mile is included in the specs for most new tires.

655 minus 600 = 55, divided by 655 = 8.4%

3.73 + 8.4% = 4.04

So the pure math answer is you need 4.04 axle ratio to have the same "feel" with the 35s as you had with the stock tires and 3.73 axle. However, the 35s are taller and wider than the stock tires, so they have more aerodynamic and mechanical drag. So you need a bit more than 4.04 to make up for that difference in drag. Therefore, 4.10 ratio should be almost perfect to get you back to the stock performance.

4.10 is a 10 percent change in ratio. 4.30 is a 15 percent change in ratio. Not a huge amount of difference. If you want more grunt off the line, or want to tug a heavier trailer up a steeper grade than the stock setup could comfortably achieve, then the 4.30 is for you.

Those same percentages will apply to engine RPM at the same corrected speed, or actual speed per GPS. So with 4.30 ratio, your speed will be 15 percent slower because of the axle ratio, but 8.4 percent faster because of the taller tires. The difference of 6.6 percent you can count on. At 70 MPH true speed with the stock tires and ratio, you will actually be going only 65.4 MPH with the 35s and 4.30. Converted to engine RPM, 70 MPH true speed with the stock setup is 2,000 RPM. 70 MPH with 35s and 4.30 ratio will be 2,132 RPM.

Not bad. If you have your speedo corrected to 600 tire revs/mile, I suspect you'll really like the new setup with 35s and 4.30 ratio. Except for the hit in MPG. The increased drag of those taller, wider tires combined with the lift is going to clobber your MPG.

But no, I don't have any seat experience with that setup.
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Old 10-26-2012, 02:14 PM   #5 (permalink)
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Thank you very much. My MPG is not that good right now. For some reason my truck is happy anything over 1900 rpm. The truck has plenty power but i have to tow at 60mph out of OD or 67+mph on OD to get my rpm's up enough for the truck to not lug. The truck is not my daily driver and my future plan is to get a 5th wheel that will be 10000+lbs. Every rear end shop I talked all recommended 4.30's. They all sid for the money 4.10 is not a big enough change. I just wanted to get some real world data and I think I just got it so thank you for all your help I go in Monday and am going to go with the 4.30's.
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Old 10-26-2012, 02:15 PM   #6 (permalink)
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Oh and yes my spedo has been corrected.
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Old 10-27-2012, 08:41 AM   #7 (permalink)
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Quote:
Originally Posted by renyren View Post
For some reason my truck is happy anything over 1900 rpm. The truck has plenty power but i have to tow at 60mph out of OD or 67+mph on OD to get my rpm's up enough for the truck to not lug.
Then you are using the wrong tune. With 8,000-pound trailer, My '99.5 needed 60-tow to be able to cruise in overdrive at 1,800 RPM without downshifting for every little bump in the road. On the plains I could usually ease down to 1,750 RPM. But with a 10,000 pound trailer, 40-tow would probably be the best tune for towing at 1,800 RPM.

Because of the HP curve with a towing tune, you engine simply doesn't make enough horses at less than about 1,800 RPM to be able to tow a heavy trailer without constant downshifting.

Towing MPG drops like a rock when the tranny downshifts from OD to direct drive. And locking out OD when towing is a sure way to kill your MPG. So for best MPG you want to tow at the lowest RPM your engine can handle without frequent downshifting for slight grades. 1600 RPM is the torque peak, so that's the goal, but you're too low on the HP curve at 1600 RPM to have enough horses hooked to pull the load. With a stock 7.3L engine, you need about 1,950 to 2,000 RPM. You say with 80-tow it's about 1,900. With 60-tow, mine was 1,800. But with a 10,000-pound 5er, I'll bet 40-tow would be right on the money to cruise at 1800 RPM.

On mine with stock final drive ratio, 1,800 RPM was 62 MPH. With 35s and 4.30, yours should be 6.6% less, or about 58 MPH in overdrive. Any faster and your MPG will drop.
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My Sierra Blanca in the sig pic was a great pickup for 11.5 years. I sold it a coupla years ago. I drove a hand-me-down 2003 F-150 SuperCrew 4.6L 2V for a while, but it was unacceptable for towing more than a rowboat. Replacement is a 2012 F-150 EcoBoost SuperCrew Lariat that tows my 5,000-pound TT like a dream.
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Old 10-28-2012, 09:13 PM   #8 (permalink)
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I have towed 12,000 lbs with my truck on 35" tires with 3.73 gears. The transmission downshifts on most hills but the truck never has any problem pulling up the hills. I towed about 10,000 lbs from Texas to Connecticut and I never felt underpowered, although it would have been nice to have those taller gears. I have an Edge Juice tuner and normally run it on level 3.

I think 4.30 would be fine for your situation. A 4.10 would give you the feel of stock RPM wise (as Smokey pointed out), but 4.30 will help you make up for the extra rolling resistance and aerodynamic drag. If I towed all the time, I would swap my gears from 3.73 to 4.10. Running 4.30 gears on 35 inch tires would be comparable to stock size tires on 4.10 gears - which is the gear ratio Ford installed in the higher towing capacity trucks.

Unloaded, I get around 15 mpg on the 35" tires with 3.73 gears. I didn't really keep track of the mileage when I was towing cross-country, but I would guess that towing 10,000 lbs on the same setup dropped me to around 10-12 mpg - I was having to stop more often for fuel, but I expected that with the load I was pulling.
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Old 12-04-2012, 03:29 AM   #9 (permalink)
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I'm glad to see you went with the 4.30's. You will be happy with them. I am running 255x85x16 and installed 4.30's a couple months ago. I could not be happier. Total curb weight is 19000-23000lbs.
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Old 12-14-2012, 01:39 AM   #10 (permalink)
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Thanks for all the help. I had 4.30's put in and so far I am very happy. Have only towed once but truck ran great and pulled good. Rpm's while towing are anywhere between 1800 and 2000 depending on speed. RPM's at 70 are 2150.
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Last edited by SmokeyWren; 12-14-2012 at 09:10 AM. Reason: fix typo
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Old 12-15-2012, 12:22 PM   #11 (permalink)
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I re-geared a few years ago, based on Smokies advise. Running 35x12.50x16.5 BFG's, and had stock 3.73's. Same issues, always searching for the right gear and rpm while towing my trailers. Re-geared to 4.10's, and ALL is good in my towing world. Good power while towing my 13,500lb toyhauler, tranny does not hunt for the right gear, and good non-towing performance.

After the re-gear, 2100 @ 71mph, right where I was before putting the 35's on, and the power rolls on right where it was designed to.

I highly recomend 4.10's with 35's
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